The basic problem to which the present invention is addressed is the classifying or sorting of particulate matter into certain size grades. The present invention is particularly applicable to mineral particulates, hereinafter referred to as granules, which are sorted so that a specific product grade granule is removed a feed stream of mineral granules. Granules larger than the product grade are sorted out and may be further processed and/or recirculated within the feed stream. The fines which are smaller than the product grade are also to be removed from the product grade granules.
It is particularly desirable to remove all, or at least as much as possible, of the fines from the product grade granules when such product grade granules are going to be subjected to further treatments or processing. Treating unwanted fines within the product grade granules increases the costs of treating and producing the product grade granules because such processing or treatments are also applied to the fines which are unusable as a product grade granules.
In one specific technology, namely the production of colored granules for use as roofing granules to be applied to roofing, such as shingles, it is highly desirable to remove the mineral fines from the product grade granules because of the expense in coating each of the granules with a specific pigment layer. Such pigments are normally applied within a ceramic coating. Moreover, specific grade granules are required for proper application of the roofing granules to roofing products, such as shingles, as such granules not only provide the aesthetic qualities to the end roofing product, but also protect the materials which comprise the roofing products, such as the asphalt-base of a shingle.
Many different methods and apparatuses are known for classifying particulate matter including dry sorting and wet sizing. The present invention is specifically directed to the field of dry screening processes and apparatuses. In a dry screening apparatus, a feed stream of particulate matter, such as mineral granules discussed above, is fed to the machine at an input end thereof, the particulate matter travels over at least one screen having a predetermined opening size (mesh), and the particulate matter that falls through the screen openings is collected for one purpose, while the material that doesn't fall through the screen is collected to be otherwise used. The particulate matter may travel over the screen under the influence of gravity, the influence of motion imparted thereto by the machine, a combination of such forces, or some other externally applied force.
When utilizing gravity, at least partially, to move the particulate matter over the screens, the apparatus must be set to dispose the screen at a sufficient angle from horizontal so that the particulate matter flows over the screen. Moreover, the screens must be pitched at an angle sufficiently steep for thinning the particulate matter after it is fed onto the screen so that the particulate matter thins and spreads out over the screen to ensure that the smaller particles are given the opportunity to pass through the screen openings. In other words, the pitch of the screen affects the rate and evenness of the traverse of the particulate matter over the screen to ensure such proper sorting. If the screens are too flat, the particulate matter becomes sluggish acting and the smaller particles are blocked from passing through the mesh of the screen by the larger particles lying on the screen, and thus the screening is ineffective.
One type of machine that has been particularly applied in the field of classifying mineral particulate matter for use in making roofing granules, is a screening machine sold by Rotex Company of Cincinnati, Ohio. Known examples includes Series 50 and 70 machines. The Rotex made machines are known to include plural screen layers, each screen layer having a different mesh size, for use in sorting mineral particulate material and specifically to remove roofing granules as product from the screening machine.
Typical roofing granules are known as 11 grade product, which means that the highest percentage of granule grade will pass through a 10 mesh (Tyler, opening size 0.065 inch, 1.68 mm) screen but will be retained on a 14 mesh (Tyler, opening size 0.046 inch, 1.19 mm) screen.
Moreover, such Rotex made machines rely on an orbital movement of the feed end of the screening machine, and specifically the screens therein. The orbital movement of each screen is substantially within the plane of each screen. Furthermore, such machines are typically set so that the plane of each screen is at an angle from horizontal primarily at about four degrees. The discharge end of such machines slides reciprocably along a substantially horizontal path as the feed end moves orbitally. Since the screens are set substantially flat, the orbital movement of each screen is responsible for dispersing the granules over the screen and traversing the granules along the screen to obtain the necessary throughput of granules. Such orbital movement at the feed end of the machine is thus relatively very substantial to ensure proper throughput of granules. A typical Series 50 model Rotex made machine having dimensions of approximately 40 inches by 120 inches moves about 3.5 inches at the feed end of the machine.
Another type of screener for sorting dry particulate materials is that using a vibration screening action. Vibration screening action means that the screens are not limited to movement substantially within the planes of each screen, but also include a component of movement in the direction perpendicular to the plane of the screens. That is, the screens are rapidly moved into and out of the plane of the screen at rest. Moreover, such vibrating screening action requires a much shorter displacement of the screens, typically about 0.625 inch, which is in the order of about 0.2 of the displacement of a Rotex type machine.
Vibration screening machines, however, require a significantly steeper angle of the screens to cause the particulate matter that is fed to the machine to be evenly displaced over the screens to ensure proper throughput of granules and fines. Typically, such machines are set at between 25.degree.-50.degree. from horizontal, although certain very light particulate materials such as plastics, for example polypropylene, may be as low as 15.degree.-25.degree.. Such machines are known to include one or more decks of vibrating screens. Known vibration screening action machines are available from Derrick Manufacturing Corporation of Buffalo, N.Y., which may be provided with one, two or three screening decks.
Heretofore, such vibration screening action machines have been found to be ineffective in the field of sorting mineral particulates, particularly for roofing granules, where it is desirable to substantially eliminate mineral fines within the product grade. Preferably, the weight percentage of mineral fines within the product grade should be below one percent. The combination of the vibrating action and the angle of the machine necessary to evenly disperse the mineral particulate generally resulted in too high a concentration of mineral fines within the product grade. In other words, as the product is taken off of the product grade defining screen deck, a substantial amount of the mineral fines was not passing through such screen. For example, when sorting 11 grade product from mineral particulate in the making of roofing granules, it was found that, in general, 1.5% or greater of mineral fines was present within samples of the 11 grade product as output of the vibration screening action machine. In contrast, the percentage of mineral fines making up such 11 grade material as product from a Rotex type machine was found to be, in general, below 1%. Of course, such percentages depend greatly on many other operating conditions which may affect the percentage of mineral fines within the product grade. Such operation conditions include the type of mineral ore, the crushing or recrushing techniques of the mineral ore before screening and the blinding or blocking of the screen mesh. Such results, however, were obtained under similar operating conditions comparing a Rotex type screening machine as described above to a vibration screening action machine available from Derrick Manufacturing Company, noted above, set at an angle of 15.degree. from horizontal.
In accordance with the conventional knowledge and understanding of vibration screening action machines, it follows that in order to improve the throughput of the mineral fines through the mesh of the screen defining the lower limit of the product grade, a steeper angle of the machine would be required to more evenly distribute the mineral particulate over each screen so that as a particulate matter traverses the screens the layer of particulate matter is sufficiently thin so that the mineral fines have ample opportunity to fall through the mesh of the relevant screen. The basic problem being that the mineral fines were not being given the opportunity to fall through the relevant screen openings because they were blocked by the larger particles which ride on the screen.